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Black and Gray Lines on Walls

DEAR TIM: I have gray and black lines on the exterior walls of my home. Some friends say the lines are caused by back drafting. Is this the real cause and what is the solution? Deborah D., Carlton, MI

DEAR DEBORAH: I think your friends watch too many movies, especially ones about the exploits of firefighters. The dark-gray lines are not back drafting at all. Back drafting happens when exhaust gases from any fuel-burning appliance are drawn backwards down a chimney. This happens when a house does not have enough combustion air entering through approved combustion air intake vents.

The lines on the walls are perfectly aligned with the exterior wall studs. The gray and black lines are created when airborne dust and dirt stick to the walls in these locations.

The dust and dirt stick to these distinct areas because of condensation. The condensation forms because the studs conduct cold through the wood and then transfer this cold to the wall surface. Water vapor in the air in your home then turns to tiny liquid droplets as the water vapor condenses on these cold strips.

You can stop the problem by lowering the humidity inside your home. Better indoor air filtration may also help the situation.

I have these lines all over our condo..along with dots and also on ceilings..there is a crawl space above..we are on the top floor and on the end of building...when where told it was our furnance.the heat exchanger had hole in it..we got new furnance ...had restoration people come in and wash down all walls and clean everything..than we had the whole place painted and here we are 2yrs later and it is all back...do you think something in crawl space needs to be fixed....more insulation...I need your help...very frustrated and upset......thank you